Before being interrupted by a real-life emergency situation, yesterday's counter-terrorism exercise at the U.S. Army Soldier Systems Center was going well, officials said.

The military complex, also known as the Natick Labs, played host to a simulated terrorist plot meant to test the Army's counter-terrorism response. "Operation Lasting Calm," as the exercise was called, introduced a scenario in which terrorists detonated bombs at points across the base, took hostages and traded gunfire with police.

The exercise unfolded in real time yesterday, as personnel at the base coordinated their responses to the scenario. Only the actors playing the terrorists and victims knew the script; those responding to counter the terrorism drama reacted as if engaged in a real situation, said David Accetta, a public affairs officer at the Natick Soldier Research, Development & Engineering Center, located on campus.

"This is realistic," he said, watching one of the "terrorists" (played by Army Sgt. Brooke Green) stalk the roof of the campus's U.S. Army Research Institute of Environmental Medicine building.

The entire exercise was scheduled to last four hours but ended about an hour early when an actual suspicious package was reported on Kansas Street, just outside the entrance to Natick Labs. Police eventually determined that the suspicious object was a propane tank and deemed it to be safe before it was hauled away to the Natick DPW yard.

Despite having to cut the exercise short, garrison commander Lt. Col. Kari Otto said she was pleased with the results.

"We met our target objectives today," she said, adding that the counter-terrorism team still had room to make improvements. "We have a whole laundry list. We'll pull everyone together and see what lessons we've learned."

The day started at about 8:20 a.m. when anti-terrorism officer Terrence Sommers, who set up the drill, gave a briefing to a roomful of actors in the campus's community building. The purpose that day, he said, was to squelch the terrorist plot with as little loss of life as possible.

"If you're from the Natick area, you know people like this installation to stay quiet," he said, explaining the exercise's name. "The point is to have lasting calm."

The drill began about an hour later when a police officer tossed an explosive canister onto a lawn across from the environmental medicine building to simulate a car bomb. On the other side of campus, another mock explosion in the Army barracks released a potentially hazardous material, triggering a lockdown of the building.

Meanwhile, first responders began scrambling to react to reports of several gunmen taking hostages in the environmental medicine building as well as in an administrative building across the street.

Throughout the exercise, the campus was essentially in lockdown mode, with access to and from the area temporarily restricted.

Page 2 of 2 - From the vantage point of a grassy hillside between the two buildings, however, the flurry of activity was barely noticeable. Most of the action was taking place indoors, and it would be more than an hour from the first explosion before snipers and SWAT teams responded.

Some of the calm on the base could be attributed to the fact that only 5 percent of personnel at the Natick Labs was taking part in the exercise - the rest were scientists, engineers and other workers who had been informed of the drill beforehand and went about their daily business. The complex primarily serves as a research and development center for the Army's clothing, armor, food and airdrop systems and other non-weapons technology.

But Accetta, who served several tours with the Army in Iraq and Afghanistan and has experience handling real-life terror scenarios, said the exercise was a relatively accurate depiction of what might happen.

"You really have to establish the situation," he said. "You don't want to go charging in - you'd put your first responders in jeopardy."

The first couple hours of the exercise unfolded behind closed doors, as the counter-terrorism team relayed information through handheld transceivers. Frequent transmissions were sent trying to identify the number of shooters, where they were, and if they were holding hostages.

As the morning progressed, Natick Police arrived to assist the effort, and eventually a Metro Law Enforcement Council SWAT team arrived at the environmental medicine building to defuse the hostage situation. Personnel in hazmat suits also decontaminated the barracks.

By noon, when reports of the real suspicious package on Kansas Street surfaced, the mock terrorist situation was essentially under control.

An hour later, Otto delivered a mock press briefing, reporting the Army had subdued three active shooters and were still tallying casualties resulting from the bomb explosions and gunfire. To wrap up the exercise, Sommers led a "hotwash" discussion back at the community building to recap the strengths and weaknesses of the response to the simulated attacks.